Circular polarizers are an important optical component in many applications, such as fiber optics, imaging lenses, and liquid crystal displays, especially in the applications when the state of polarization is desired to be independent of the azimuthal angle of the incident light, or to be independent of the azimuthal angle of the slow axis of anisotropic media. Circular polarizer can be used in the studies of the properties of thin films, as described in to U.S. Pat. No. 6,219,139 issued to Lesniak on Apr. 17, 2001. Circular polarizers are also used in Liquid crystal displays (LCDs), as described in U.S. Pat. No. 6,549,335 B1 issued to Trapani et al. on Apr. 15, 2003, U.S. Pat. No. 6,583,833 B1 issued to Kashima on Jun. 24, 2003, U.S. Pat. No. 5,796,454 issued to Ma on Aug. 18, 1998, and U.S. Pat. No. 6,628,369 B2 issued to Kamagal on September, 2003.
Circularly polarized light is a polarized plane light with equal magnitude in its orthogonal components and the phase difference between the orthogonal components is ±π/2. Circularly polarized light can be generated by cholesteric liquid crystal, according to U.S. Pat. No. 5,796,454, or can be converted from linearly polarized light with a linear polarizer and a quarter-wave plate, of which the principal optical axes is 45 degrees with respect to the transmission axis of the linear polarizer, according to U.S. Pat. No. 6,788,462 B2 issued to Lesniak on Sep. 7, 2004. According to U.S. Pub. No. 2004/0109114 A1 filed on Jun. 10, 2004, a circularly polarized light can also be converted from a linearly polarized light with a substrate having a longitudinal direction and the combination of a half-wave plate and a quarter-wave plate, where the principal optical axes of the half-wave plate and the quarter-wave plate are +30 and −30 degrees with respect to the longitudinal direction of the substrate, respectively. A quarter-wave plate is an optical anisotropic element which induces π/2 phase difference between the orthogonal components of the light passing through. A half-wave plate is an optical anisotropic element which induces π phase difference between the orthogonal components of the light passing through.
However, when cholesteric liquid crystal is used, the induced circularly polarized light sustains blue shift at oblique incident angle. Furthermore, the fabrication of cholesteric liquid crystal cell is difficult. When the combination of a linear polarizer and a quarter-wave plate or using the combination of special substrates having a longitudinal direction and a half-wave plate together with a quarter-wave plate, the quarter-wave plate or half-wave plate only induces π/2 or π phase change at normal incident angle. At oblique incident angles, the phase change is varied with both incident angle and the azimuth of incident plane, which results in elliptically polarized light instead of the desired circularly polarized light.
FIG. 1 shows the structure of a conventional right-hand circular polarizer comprising one linear polarizer and one quarter-wave plate. Along the propagation direction of the incident light, the structure comprises one linear polarizer 101, and one quarter-wave phase retardation film 102. As shown in FIG. 2, at 85° incident angle and 45° azimuth of incident plane, the S3 of the state of polarization emerging from such a conventional circular polarizer is −0.829. However, the S3 of the desired right-hand circular state of polarization is −1. Therefore, the difference between the S3 of the desired circular state of polarization and the S3 of the state of polarization emerging from the conventional circular polarizer is 0.171 at 85° incident angle and at 45° azimuth of the incident plane. As a result, the optical performance for those applications having a wide range of incident angles is greatly deteriorated. For example, in a liquid crystal display, the non-ideal circular state of polarization emerging from conventional circular polarizer reduces the contrast ratio of the LCD at wide incident angles. Therefore, a need exists for a wide-acceptance-angle circular polarize for use in these applications.